1. Field of the Invention
The present invention relates to a liquid crystal display or more specifically to a multiplex-driven liquid crystal matrix display.
2. Description of the Prior Art
Due to the recent increasing demand for a liquid crystal display with larger display information capacity, the industry's attention is gradually moving from a segment display to a matrix display. For diversification of the information displayed on an matrix display, the increase in the matrix-driving multiplex frequency (the number of scanning electrodes) is demanded.
A transmission type liquid crystal display (TN-LCD) which incorporates color filters or color polarizers for colored display is increasingly drawing attention. To realize a liquid crystal color television with this display system, investigation into the drive method, color filter construction and suitable liquid crystal material has been conducted in various sectors of the industry. The primary challenge for this display system is to produce colors of high purity and wide range of hues. However, study in this field has not been sufficient so far.
When a liquid crystal X-Y matrix display panel is driven by the optimal voltage averaging method with a multiplex frequency of N, it has been well-known that the maximum contrast ratio is obtained when there exists the relationship as expressed by the following equation (1), between the scanning pulse peak voltage V.sub.1 and the signal pulse peak voltage V.sub.2. EQU V.sub.1 =.sqroot.NV.sub.2 ( 1)
In this case, the ratio .alpha. of the effective voltage for lighted-on picture elements, V.sub.ON, to that for the lighted-off picture elements, V.sub.OFF, is expressed by the equation: ##EQU1##
It is clear from the above formula that the difference between V.sub.ON and V.sub.OFF reduces as the number of scanning electrodes N increases.
Assuming that N is 200, for instance, the voltage applied to the lighted-on picture elements is only 7.3% higher than that applied to the lighted-off picture elements. When N is larger, the voltage drops due to the electrode resistance, and the threshold voltage for the electric optical properties such as lighting-on and -off of the display panel-constituting liquid crystal fluctuates, causing less uniform or poorer contrast of a picture of the liquid crystal display panel. Accordingly, the number of scanning electrodes N cannot be increased without deteriorating the picture contrast.
Moreover, in the multiplex-driven colored display, dependence of the transmitted light intensity upon the voltage applied to the liquid crystals varies with the color (light wavelength) of the transmitted light. Even if other properties of the display panel-constituting liquid crystals are uniform, therefore, it is difficult to achieve a good color balance.